This invention relates to ultra-violet (UV) detectors, and in particular visible-blind UV detectors comprising II-VI alloys as the active material.
UV detectors with high responsivities for wavelengths shorter than 400 nm are very important for applications such as flame monitoring, pollutant detection and UV astronomy, as well as for advance medical techniques and instrumentation. Conventionally two types of devices have been used for UV detectors: photo-multiplier tubes and silicon p-i-n photodiodes.
Conventional photo-multiplier tubes have been satisfactory in the past, but in an increasing number of applications silicon p-i-n photodiodes have been preferred because of their low size and low voltage operation in spite of having a lower quantum efficiency. In recent years there has also been an increasing interest in developing GaAlN alloys and SiC thin films for visible-blind and solar-blind UV detection applications. Compared to SiC, GaAlN has a number of advantages: (1) it is a direct bandgap material and hence has a higher absorption coefficient, (2) it has a sharper cut-off, and (3) the doping control in the material is sufficiently well-developed that heterojunction devices are now possible, which will result in improved quantum efficiency. In addition the band-gap energy of GaAlN alloys spans the range from 3.4 to 6.2 eV making it possible to tailor the detector to have a specific long-wavelength cut-off between xcx9c3600 xc3x85 and xcx9c2000 xc3x85. Several GaNxe2x80x94GaAlN based photoconductors and photodiodes of both Schottky and p-i-n junction types and having good performance have been successfully fabricated. More recently still a high gain GaN/AlGaN heterojunction phototransistor has been demonstrated. The highest external quantum efficiency achieved is around 70% at 355 nm.
However a problem with previous semi-conductor based UV photodetectors is the lack of a suitable lattice-matched substrate. The most commonly used substrate, sapphire (A12O3), is lattice-mismatched to the nitrides by xcx9c14% and has a thermal expansion coefficient that is almost twice as large as that of GaN. As a result, a high density of misfit dislocations and traps is inevitably present in these structures which severely limits the response time of the detectors. For example the fastest reported response time for a 250xc3x97250 xcexcm GaN photodiode is still of the order of microseconds. It is also difficult to integrate nitride-based photodetectors with Si technology. The lattice mismatch between GaN and Si is even greater (18.7% between cubic GaN and Si) and thus so far there are no reports of good performance GaN based devices grown on a Si substrate.
According to the present invention there is provided a UV detector comprising ZnS1xe2x88x92xTex alloy as the active material. The proportion of Te in the alloy may be varied to provide good lattice matching to a substrate and will depend on the substrate chosen, but preferably 0xe2x89xa6xxe2x89xa60.1. Possible substrate materials are doped or undoped GaAs, Si or GaP. The active layer may de a doped layer, for example n-type doped ZnSTe:Al, or may be a layer of intrinsic ZnSTe.
In a first preferred embodiment the detector is a Schottky barrier structure comprising: (a) a substrate layer of n+-type Si, GaAs or GaP, (b) an active layer of doped or undoped ZnS1xe2x88x92xTex formed on a first upper surface of the substrate layer, (c) a layer of a first conducting material formed on the surface of the active layer as a first electrode, and (d) a second conducting material formed on a second lower surface of the substrate as an Ohmic contact.
Preferably an upper surface of the detector is formed with an anti-reflection coating. This increases the quantum efficiency of the detector by minimising reflection losses.
In some circumstances this structure may however have a relatively low quantum efficiency caused by the Ohmic contact being located beneath the substrate and because carriers may therefore be trapped at the junction between the substrate and the active layer. Therefore in a more preferred embodiment the detector is a Schottky barrier structure comprising: (a) a substrate layer of GaAs, Si or GaP, (b) a first layer of Al doped ZnS1xe2x88x92xTex formed on a first upper surface of the substrate layer, (c) a second active layer of ZnS1xe2x88x92xTex formed on the surface of the first layer and only partially covering the first layer so as to leave a part of the first layer exposed, (d) a layer of first conducting material formed on the surface of the second active layer so as to form a first electrode, and (e) an Ohmic contact formed on the exposed part of the surface of the first active layer.
Preferably this structure is made with a substrate of Si or GaP. The material for the first electrode is preferably gold, while the Ohmic contact may be formed of at least one indium pellet. With a substrate of Si the Te composition may be such that x=0.03, while if the substrate is GaP preferably x=0.06, in both cases x being chosen to achieve good lattice matching between the substrate and the active layer.
This embodiment of the invention may be formed by (a) depositing a first layer of Al doped ZnS1xe2x88x92xTex by molecular beam epitaxy on a first upper surface of a substrate of GaAs, Si or GaP, (b) depositing a second active layer of ZnS1xe2x88x92xTex by molecular beam epitaxy on the surface of the first layer, (c) removing a portion of the second active layer by means of chemical etching to expose a part of the first layer, (d) depositing a first conductive material on the second active layer as a first electrode, and (e) forming an Ohmic contact on the exposed part of the first layer.
However, the wet chemical etching can have deleterious effects and so another method of forming the detector is by (a) depositing a first layer of Al doped ZnS1xe2x88x92xTex by molecular beam epitaxy on a first upper surface of a substrate of GaAs, Si or GaP, (b) applying to a part of the surface of the first layer a protective material to cover the part of the surface, (c) depositing a second active layer of ZnS1xe2x88x92Tex by molecular beam epitaxy on the surface of the first layer not covered by the protective material, (d) depositing a first conductive material on the second active layer as a first electrode, and (e) forming an Ohmic contact on the exposed part of said first layer.
In addition to ZnSTe-based UV photodetectors, the present invention also relates to UV photodetectors comprising other II-VI alloys as the active material, and in particular to a UV photodetector comprising ZnS1xe2x88x92xSex as the active material. The ZnSSe photodetector may be formed using the same techniques and with the same structure as for the ZnSTe-based photodetectors. Again, the proportion of Se may be chosen to provide good lattice matching with the substrate material, but preferably 0xe2x89xa6xxe2x89xa60.5, and more preferably still x=0.5.